The invention relates to a luminaire with an array of light emitting diodes (LEDS), and more particularly to a white light emitting luminaire with a control system for adjusting the individual components to maintain a desired color balance (chromaticity).
U.S. Pat. No. 5,301,090 discloses an LED luminaire having an array of LEDs including a plurality of LEDs in each of the colors red, green and blue. The LEDs for each color are wired in parallel and provided with a separate power supply, and a diffusion screen is provided over the array. The chromaticity of the assembly is manually controlled by three knobs for the respective colors.
LEDs are not uniformly bright; for a given drive current, light output varies from chip to chip, and also varies over the life of each chip. Light output also varies inversely with temperature, but not uniformly for each color. Finally, in a block of LEDs of a given color, the light output will vary if one or more of the LEDs fails. Given all the factors, which can affect the color balance of any array of LEDs, it would be desirable to automatically monitor and regulate the color balance, especially in a white-light emitting luminaire.
It is known to control current to an array of LEDs in a given color based temperature, for example in a traffic light. This scheme would be cumbersome in a luminaire having LEDs in a plurality of colors, because the temperature (and therefore the light intensity) does not vary uniformly for the various colors.
U.S. Pat. No. 6,127,783 describes monitoring the color balance of a white luminaire composed of red, green, and blue LEDs, by separately reading the red, green, and blue channels with a single photodetector. During each measurement cycle, two colors are turned off for a period of milliseconds, while the intensity of the third color is measured. This technique was found to cause perceptible flicker.
Previously filed U.S. patent application Ser. No. 09/663,050 attempts to solve this problem by increasing the intensity of other colors slightly just before and after they are turned off. However, perceptible flicker still existed, introducing discomfort for the user.
It would be desirable to automatically control the chromaticity of a white light emitting luminaire, without regard to the factors which cause the light outputs of the individual colors to vary.
It would further be desirable to automatically control the chromaticity without resorting to a spectrally resolving light measuring system such as a photodiode and filter for each of the respective colors.
It would be additionally desirable to eliminate any perceptible flicker during the measurement cycle eliminating discomfort for the user.
According to the invention, the combined light output (chromaticity) of a white light emitting LED luminaire is electronically controlled based on measurements by a single photodiode arranged to measure the light outputs of at least a plurality of the LEDs in the array. This is accomplished by measuring the light output of the LEDs in each color separately in a sequence of time pulses. For an array of red, green, and blue LEDs there are three time pulses in a measuring sequence. During each time pulse, the current for the color being measured is turned off. The response time of a typical photodiode is extremely short, so the measuring sequence can be performed in a sufficiently short time that an observer will not detect it (e.g. 10 ms).
Measured light outputs for the colors are compared to desired outputs, which may be set by user controls, and changes to the power supply for the color blocks are made as necessary. Chromaticity is thus automatically controlled without regard to the factors which may cause it to change. The user inputs permit varying the desired chromaticity to either warm white (more red output) or cool white (more blue output).
In order to best compensate for temperature dependent changes during a warm-up phase, the electronic control circuitry may undertake the measuring sequence more frequently during warm-up. Less frequent measurements are sufficient to compensate for long term changes in the LEDs after a stable operating temperature is reached.
Where the LEDs in each color are wired in parallel, varying the current to the remaining LEDs during the next measuring sequence can automatically compensate the failure of an LED.
In accordance with one embodiment of the invention, the array of LEDs is driven by a current supply source that includes a measuring drive pulse having at least a xe2x80x9cturn-offxe2x80x9d portion. The LEDs in each color have a light output that has a nominal continuous value during ordinary operation and is interrupted during the xe2x80x9cturn-offxe2x80x9d portion. The array of LEDs has a combined light output when current is supplied by the current supply source. A photodiode takes two measurements. First, the photodiode measures the light outputs of all LEDs in the array. Second, the electrical current is selectively turned-off to one of the LEDs so that the photodiode measures the light output for the remaining colors in response to the measuring drive pulse. Measurement of the light output of the LED is determined by the difference between the first light level and the second light level.
These and additional advantages of the invention will be apparent from the drawing and description which follows.